I'm not overclocking anything. CPU is running at stock (only "OC" feature turned on is Asus Multi-Core enhancement). Memory is running at 1.53V (bumped up from 1.5V to try to fix the problems) and 9-9-9-24-2T, which is what it is rated for. Fresh install of Windows 8 Pro. I've tried builtin drivers as well as the latest ones from Asus.

The random nature of the problems makes me lean toward memory issues, but I've run HCI memtest, Windows memory diagnostics, and Prime95 with no issues. I'm just getting ready to run memtest86 for a while. Any other ideas?

Disable Multi-core Enhancement. You are still overclocking the CPU with Multi-core Enhancement because the cores in your CPU are running at the highest turbo speed but it is not within spec to be running all the cores at the single core turbo speed. I have an Asus P9X79 Pro motherboard and Mult-core Enhancement is turned on by default if XMP is enabled even though there is no Multi-core Enhancement setting in the BIOS but I know it's turned on by default if set to XMP because it says that all cores turbo to the single core speed of my CPU. Change the core turbo speeds to "per core" and set them to Auto, and that way all the cores should be running at their spec'd turbo speeds. Also running memtest 86+ is a good idea which you say you will be running.